Biofouling is the result of marine organisms settling, attaching, and growing on submerged marine surfaces. The biofouling process is initiated within minutes of a surface being submerged in a marine environment by the absorption of dissolved organic materials which result in the formation of a conditioning film. Once the conditioning film is deposited, bacteria (e.g. unicellular algae) colonize the surface within hours of submersion. The resulting biofilm produced from the colonization of the bacteria is referred to as microfouling or slime and can reach thicknesses on the order of 500 μm.
Biofouling is estimated to cost the US Navy alone over $1 billion per year by increasing the hydrodynamic drag of naval vessels. This in turn decreases the range, speed, and maneuverability of naval vessels and increases the fuel consumption by up to 30–40%. Thus, biofouling weakens the national defense. Moreover, biofouling is also a major economical burden on commercial shipping, recreational craft, as well as civil structures, bridges, and power generating facilities.
Any substrate in regular contact with water is likely to become fouled. No surface has been found that is completely resistant to fouling. Due to the vast variety of marine organisms that form biofilms, the development of a single surface coating with fixed surface properties for the prevention biofilm formation for all relevant marine organisms is a difficult if not impossible task.
Anti-fouling and foul-release coatings are two main approaches currently used for combating biofilm formation. Anti-fouling coatings prevent or deter the settling of biofouling organisms on a surface by the use of leached biocides, typically cuprous oxide or tributyltin, into the water. The biocides are either tethered to the coated surface or are released from the surface into the surrounding environment. Use of these types of coatings has caused damage to the marine ecosystem, especially in shallow bays and harbors, where the biocides can accumulate. As such, the use of tributyltin has been banned in many parts of the world. These products are effective for only approximately 2 to 5 years.
Foul release coatings present a hydrophobic, low surface energy, and resulting slippery surface that minimizes the adhesion of the biofouling organisms. The most commonly used and highly successful of these is a nontoxic silicone-based paint. The silicone-based coating requires several layers to make it effective, and therefore it can be quite costly. Effectiveness lasts up to 5 years at which time recoating may become necessary. These products are considered to be more environmentally sound as compared to anti-fouling coatings because they do not leach toxins. However, they are subject to abrasion, and therefore their use is limited to areas that are not susceptible to damage caused by ice or debris.